In many applications it is important to be able to quickly and accurately locate objects such as pipes, cables, mines, and barrels that are buried beneath the surface of the earth. Such objects may be located using Ground Penetrating Radar (“GPR”) techniques in which electromagnetic waves are transmitted into the ground and reflected. (The term ‘ground’ includes soil, concrete, asphalt, and the like.) The reflections are analyzed according to methods that are well known in the art to determine the location of any object that may be buried there beneath. Other methods well known in the art may also be used, including sonar techniques and inductive techniques.
GPR techniques use transmitting antennas to emit the electromagnetic waves that propagate into the ground and interact with the buried objects. This interaction results in a scattered wave, which is measured by the receiving antenna of the GPR device. By changing the location of the transmitting antenna and recording the corresponding signal that is received and then output by the receiving antenna as a function of time (or frequency) and location, one obtains the radar data from which the information about the buried objects may be extracted. However, for the radar data to be useful, the positions of the measurement locations must be accurately known. Further, it is important for cost efficiency that the system be able to cover a large area in a short period of time; that is, it is important that the system have a high survey speed, which means that the antenna must travel at a high speed.
Such high speed movement may create undesirable mechanical stresses on the antennas. For example, in the simplest scanning system the antenna moves linearly back and forth across the width of the scanning area. This back and forth linear motion requires that the antenna slow down, stop, and then speed up each time it reaches the edge of the scanning area. This type of back and forth movement creates tremendous mechanical stresses in the antennas and survey system when the system is operated at a high survey speed. In fact, these extreme stresses severely limit the survey speed that is obtainable with a linear scanning system.
It is an object of the current invention to provide a rotating GPR system for which the antenna speed and survey speed can be very high without causing excessive mechanical stresses on the antennas and scanning system. With a rotating system, the antennas do not have to slow down when they reach the edge of the scanning area, but instead may operate at a constant speed. As a consequence, the mechanical stresses are much less for a rotating GPR system than for a linear GPR system.
A rotating GPR system is described in U.S. Pat. No. 4,967,199 (“Ground Probing Radar Method and Apparatus”) to Gunton et al. and in Surface Penetrating Radar by D. J. Daniels (IEE Press, 1996, pp. 200–204). Those references describe a system in which the antennas are interleaved spirals whose axes correspond with the axis of rotation. Further, unlike the present invention, the rotation in these systems is used solely to reduce clutter rather than to move the antenna system along the ground.